The present invention relates to novel photohardenable compositions and to photosensitive materials employing them. More particularly, it relates to free radical addition polymerizable compositions containing a cationic dye-borate anion complex as a photoinitiator.
U.S. Pat. Nos. 4,399,209 and 4,440,846 to The Mead Corporation describe imaging materials and imaging processes in which images are formed through exposure-controlled release of an image-forming agent from a microcapsule containing a photohardenable composition. The imaging material is exposed image-wise to actinic radiation and the microcapsules are subjected to a uniform rupturing force. Typically the image-forming agent is a color precursor which is released image-wise from the microcapsules to a developer sheet whereupon it reacts with a developer to form a visible image.
Full color imaging materials include a photosensitive layer which contains three sets of microcapsules. Each set of microcapsules is sensitive to a different band of radiation in the ultraviolet or blue spectrum and contains a cyan, magenta or yellow image-forming agent. One of the problems which has been encountered in designing commercially acceptable full color imaging materials employing these techniques has been the relatively narrow wavelength band over which most photohardenable compositions are sensitive to actinic radiation. In most cases, te compositions are only sensitive to ultraviolet radiation or blue light, e.g., 350 to 480 nm. Furthermore, the absorption spectra of the initiators employed in these microcapsules are never perfectly distinct. There is always a degree of overlap in the absorption curves and sometimes it is substantial. Exposure conditions therefore must be controlled carefully to avoid cross-exposure.
It would be desirable to extend the sensitivity of the photohardenable compositions used in these imaging materials to longer wavelengths. By extending the sensitivity of the photohardenable compositions to longer wavelengths, the absorption bands of the initiators can be separated and the amount of overlap in the absorption spectra of the initiators and the concomitant incidence of cross-exposure can be reduced. It would be particularly desirable if compositions could be designed with sensitivities to selected wavelength bands throughout the visible spectrum (400 to 700 nm) since this would provide a visible light-sensitive material which could be exposed directly by reflection or transmission imaging and without the need for image processing to translate the image into three wavelengths of ultravoilet or blue radiation.
Visible light-sensitive, dye-sensitized photopolymerizable compositions are known in the art. A survey of these systems is provided by Eaton, David E., "Dye Sensitized Photopolymerization", Advances in Photochemistry, Vol. 13, pp 427-87 (1985). In their simplest form these compositions include a photopolymerizable vinyl compound, a photoreducible or photooxidizable dye and an activator which functions as a reducing agent or an oxidizing agent for the dye. The dyes are excited by light to a triplet state which reacts with the reducing agent or oxidizing agent to yield radicals which can initiate polymerizations. There are a number of examples of dye-sensitized photopolymerizable compositions in the patent literature. See Oster, U.S. Pat. Nos. 2,850,445 and 2,875,047 (Rose Bengal with hydrazine or thiourea); 3,547,633 (quinoidal family dyes with triorganophosphines, triorganoarsenines or sulfinic acid derivatives); 3,615,452 (phenazine or oxazine dyes and diazonium salts); 3,627,656 (phenothiazine dyes and sulfinic compounds); 3,495,987 (cyanine dyes and bromine donors) and 3,488,269 (thionine dyes and methylenes or methines).
U.S. Pat. No. 3,579,339 describes panchromatic imaging systems in which a dye-sensitized photopolymerizable composition containing a color coupler of the type used in silver halide color films is dispersed in a binder such as gelatin and coated on a support to provide a photosensitive layer having small, dye-sensitized, photopolymerizable droplets therein. Exposure to light polymerizes the composition and reduces the permeability of the droplets to a liquid color developer such that application of the color developer differentially produces color in the unexposed or underexposed droplets. Three photopolymerizable dispersions are provided on a support in three separate layers. One layer contains a yellow color former and is sensitive to blue light, the second layer contains a magenta color former and is sensitive to green light and the third contains a cyan color former and is sensitive to red light. Exposure to visible light selectively hardens the droplets such that, upon application of a developer, cyan, magenta and yellow positive images are formed in the respective layers. These systems are described in more detail in Chang et al, "Color Print Systems Based on Dispersion Photopolymerization", Photographic Science and Engineering, Vol. 13, No. 2, pp. 84-9 (1969).